Hello! Just started ME3 as a Vanguard (imported from ME2) and I'm on Mars.
I haven't been using Nova too much yet because my shield goes down very quickly and hence I'd take health damage.
In this game, I can't heal all health damage by waiting behind cover, only 1 bar can be healed.
So... Anyone has tips on how to use Nova effectively?
It's a shame you can't one shot enemies anymore :/
I like to charge one shot rinse repeat in ME2 
A few things:
- When using Nova, you are invulnerable to damage for 1.5 seconds. (Note that you can still be sync-killed during Nova from Brutes, Banshees, and Phantoms.)
- You can Nova-cancel by rolling or using melee during the up portion of Nova. Nova-cancelling stops you from using Nova, but you still get the invulnerability duration (1.5 seconds).
- Take Half Blast at rank 5 so you can Nova twice. This means you can be invulnerable for up to 3 seconds.
Nova works best when you Charge> Nova,or if you Charge > Shoot > Nova. With Half Blast you can Nova twice so you can Charge > Nova > Nova, or Charge > shoot > Nova > Nova.
Also of note: Charge gives you +50% damage reduction for 4 seconds after charging. So you generally want to keep your weapon weight light enough so you keep Charge on 4s or quicker cooldown. (3 seconds if you want to exploit Half Blast Nova).
There are some topics here you may want to check out: Mass Effect 3 Single Player: A compilation of strategy guides and discussions. Of note:
For you concern about one-shotting shielded enemies, this is based on a mechanic called shieldgate, explained here. But generally you do not need to worry about it if you carry a good rapid-firing weapon (like the Punisher or Hurricane SMGs) or weapons that fire multiple bullets at once (like most shotguns and the Talon).
Good weapons to use on a Charge-Nova Vanguard:
- Claymore
- Reegar (DLC)
- Wraith
- Piranha (DLC)
- Punisher SMG (DLC)
- Hurricane SMG (Collectors Edition or modding)
- Talon
Recommended build:
Incendiary Ammo: Squad Bonus - Headshot Damage - Explosive Burst
Biotic Charge: Radius - Weapon Synergy - Barrier
Nova: Damage - Half Blast - Pierce
Assault Mastery: Damage - Weight Capacity - Shotgun Damage
Fitness: Durability - Melee Damage - Durability
Bonus powers:
The following are good bonus powers for the Vanguard:
- Fortification. Good for damage reduction, and power synergy for increasing Incendiary Ammo damage.
- Reave: for priming biotic detonations on your own.
- Lash, for priming biotic detonations, and at rank 6 can pull enemies with shields or barriers (as long as they do not have armor).
- Energy Drain if you really are concerned about shields; you should not be that concerned about shields in this game.
Squadmates:
- Liara for priming biotic explosions with Singularity and Warp and Stasis.
- Javik for priming biotic explosions with Pull/Slam and Dark Channel and Lift Grenades
- Kaidan for priming biotic explosions with Reave
EDI, Ashley, and Tali are good for priming various tech explosions which you can detonate with Nova or Charge.
James is rather weak as a squadmate for the Vanguard.
(I am probably forgetting a squadmate. Oh well.)
One last thing:
Damage protection: you actually take about +40% more damage when out of cover. So that +50% damage protection for 4 seconds from biotic Charge means you are only getting +10% damage protection when out of cover for 4 seconds after charging.
This is why taking Fortification as a bonus power can be good for giving you always-on damage protection. Reave can also give you damage protection for a short time. Both Fortification and Reave can give you up to +40% damage protection. Barrier and Defense Matrix can only give you a maximum of +30% damage protection.